This invention relates to electromagnets for use in magnetic resonance imaging (MRI) systems and in particular to magnets which are open, i.e., magnets in which an imaging volume wherein that portion of a patient being imaged is positioned, is not surrounded by the magnet.
A special requirement for MRI is a strong uniform magnetic field, typically 0.2 to 2 Tesla, with a field homogeneity of a few parts per million in the imaging volume, typically a sphere of 30 cm to 50 cm in diameter. Most commonly such a field is produced by an electromagnet having a solenoid construction but this necessitates a patient being surrounded by the magnet and enclosed within a tube, which can cause a feeling of claustrophobia. However by using open magnets, to which this invention particularly relates, this problem is overcome.
Open electromagnets for use in MRI systems are well known. One known form of electromagnet comprises a pair of juxtaposed magnetic poles of opposite polarity between which the imaging volume is defined, which poles are linked and supported by a yoke which provides a magnetic flux return path and which principally comprises a generally C-shaped iron frame. Because large amounts of iron are required, these known C-shaped magnets are very heavy, especially for high field magnets which require many tonnes of iron to define the flux return path.
Another known form an open electromagnet, often described as a `split pair`, comprises a pair of juxtaposed sets of coils, which are generally of a solenoid construction and may include a bore tube around the axis. The sets of coils are held apart by a support structure so that access may be gained to the imaging volume between them along any of the principal axes of the system. Such a system is described in U.S. Pat. Ser. No. 5,381,122 and in a paper by Laskaris et al, entitled `A Cryogen-Free Open Superconducting Magnet for Interventional MRI Applications` published in IEEE Transactions in Applied Superconductivity, Volume 5, Number 2, June 1995.
The magnetic force between the juxtaposed magnetic poles of known open magnets is very large, and accordingly this imposes large forces on a structure used to support the poles. Moreover, in superconducting electromagnets, wherein field windings which define the poles are contained in a cryostat, mechanical design problems arise due to the requirement for supporting, within the cryostat, windings which are subjected to this very large magnetic force. In known `split-pair` systems, this problem is overcome by arranging that the supporting structure which resists this magnetic force is connected directly between the windings and is therefore maintained at the same low temperature as the windings. However this requires large structural components which extend across the gap between the juxtaposed poles, severely restricting the openness of the system and requiring a very complex and expensive arrangement for the cryostat.
An alternative approach is to transmit forces acting on the cryostatically contained juxtaposed windings mechanically to `warm` parts of the clryostat structure, and to provide a support frame which serves to hold the two cryostats apart. Conventionally, `cold` components within a cryostat are supported by means of rods or bands made from material with a low thermal conductivity and high strength, such as glass fibre reinforced plastics material. However the magnetic forces between the poles are many times greater than forces due to their weight, with the result that supporting bands or rods must have a much larger cross-section to transmit these forces and consequently they allow a much larger flow of heat into the `cold` parts of the system. In a system cooled by liquid helium this results in a high rate of evaporation of the liquid, and in a system cooled by a cryogenic refrigerator this results in excessive power consumption.
It is known that the use of large amounts of iron in close proximity to the coils will substantially modify the forces acting on the coils. For example, in Danby et al, PCT WO88/04057, the forces on the coils in an iron shielded magnet are reduced by more than an order of magnitude by suitable positioning of the coils relative to the iron. However, this does not address the weight issue, which in the case described results in a system weighing in excess of 100 tonnes in order to limit the stray field to a reasonable value.